The present invention is directed to a conveying unit for conveying flat objects. The conveying unit includes first and second endless belts and a roller.
Conveying units are employed in folding apparatus for conveying signatures. These signatures have previously been cut off a web of imprinted material.
The signatures each can consist of a variable number of sheets which sheets are not connected with each other. In the course of conveying the signatures, it is therefore of great importance that the two endless belts and the roller move at speeds which are exactly matched to each other in order to prevent shearing forces from acting on the signatures that are clamped between them. Such shearing forces might lead to deformation and fanning of the signatures in the course of their conveyance.
In conventional conveying units, the movement of the second endless belt is coupled, via a transmission gear with a fixed transmission ratio, to the rotation of the roller. The first endless belt, which partially loops around the roller, is driven by the roller by friction. Therefore, the path speed of the first belt corresponds to the circumferential speed of the roller if no objects are conveyed between them. If conveyed objects are located in the loop area between the roller and the first belt, this has an effect on the speed of the first belt which is similar to an increase in the diameter of the roller. The speed of the first belt therefore increases with the thickness of the objects to be conveyed. The speed of the second belt is constant. This has the result that only with a definite thickness of the objects to be conveyed do the two belts run exactly the same, so that the objects to be conveyed are not subjected to shearing forces.
DE 42 41 810 A1 discloses a folding apparatus wherein first and second belts are arranged between the cutting cylinder and the folding cylinder. These belts are driven together by a common motor.
The object of the present invention is directed to providing a conveying unit for flat objects.
In accordance with the present invention, this object is attained by the provision of first and second endless belts and a roller. A first conveying path section extends between the first endless belt and the second endless belt. A second section of the conveying path extends between the first endless belt and the circumference of the roller. The first endless belt and the second endless belt are driven independently of each other.
The advantages which can be attained by the present invention reside, in particular, in that it is always possible to produce the same speed between the two endless belts, even with differences in the thickness of the objects to be conveyed, so that the objects can be conveyed gently and free of shearing forces.
The conveying unit has a drive mechanism for the second belt which can be regulated independently of the speed of rotation of the roller.
This drive mechanism is preferably associated with a regulating device that is acting to match the speeds of the two belts. This regulating device preferably regulates the speed of the second belt proportionally to the speed of rotation of the roller by the use of a variable proportionality factor. By matching the proportionality factor as a function of the thickness of the objects to be conveyed, it is possible to assure that objects of variable thickness are conveyed free of shearing forces.
To determine the proportionality factor, the regulating device can be connected to a sensor for measuring the speed of the first belt. The speed of the first belt varies linearly with the thickness of the conveyed objects. Freedom from shear forces acting on the conveyed objects can be achieved by a simple matching of the speed of the two belts.
It is also within the scope of the present invention to couple the regulating device to a sensor for detecting the shearing deformation of the conveyed objects. For example, such a sensor can be implemented as a camera with a connected electronic image processor. It is also possible for an operator to manually vary the proportionality factor on the basis of a detected shear deformation of the conveyed objects.
Another option is to couple a sensor for detecting the thickness of the objects to the regulating device. Such a sensor can be particularly arranged upstream of the input of the regulating device. This makes it possible to match the belt speeds of the conveying unit to a change in thickness of the object before the object, on which the thickness measurement had been performed, reaches the conveying device.